1. Field of the Invention
The present invention relates to control systems for thermoelectric coolers (TECs) and more particularly relates to an efficient, small form-factor controller for two or more large TECs where the electrical power demand of each can exceed 200 watts.
2. Description of Prior Art
Thermoelectric coolers or Peltier devices have been applied to many uses, as varied as temperature control of semiconductor lasers devices and thermal imaging systems to portable refrigeration systems for automotive applications. In the case of the former, there is prior art embodied in a number of patents for systems that provide precise temperature control. Specifically, U.S. Pat. No. 5,088,098 of Muller, et al. teaches a simple temperature control system that utilizes pulse width modulation (PWM) of the electrical current supplied to the TEC device as the means to control rate at which the TEC device adds or removes thermal energy from the system requiring temperature control. This concept is further refined in U.S. Pat. No. 5,450,727 of Ramirez, et al. with the implementation of a temperature control feedback system wherein the error signal developed from the difference between a control input and the device temperature is the control input for a PWM current source. Finally, a closed loop TEC temperature control system with still further refinements for improved temperature regulation is taught in U.S. Pat. No. 6,205,790 of Denkin, et al. This patent claims advantages in the temperature control over prior art designs by limiting the maximum feedback error signal amplitude and zeroing the feedback error signal at operating point crossover. The cited prior art demonstrates significant advancement in the application of TEC devices to temperature control of small devices, e.g. semiconductors, wherein a single TEC device has sufficient thermal energy transfer capacity to control the system temperature.
When human beings are deployed to working environments with extreme temperatures, their capacity for physical labor and mental acuity may be greatly diminished because of the effect of very miniscule changes in body core temperature. This is especially evident when humans engage in underwater diving where the water temperature is only a few degrees warmer than normal body temperature. As the body absorbs thermal energy from the warm ambient environment, the diver will quickly become nauseous, disoriented and at risk of death. There is therefore a need to provide control of the diver's body core temperature on a real time basis. A closed-loop perfusion system with a working fluid may be employed to conduct excess heat away from the human body, and a heat pump system in the perfusion loop can then transfer the excess heat to the ambient environment. TEC devices are the solution of choice for the heat pump system in this application because of their simplicity, ruggedness and ability to both heat and cool without system reconfiguration.
For a temperature control system where the human body is immersed in water with an ambient temperature of +40° C., the required cooling capacity may be as high as 500 watts or more. Because the maximum cooling efficiency of TEC devices in this application is limited to approximately 50%, the total input power demand of the TEC devices may exceed 1000 watts. In one TEC system already constructed and demonstrated by applicant, the prime power voltage was 22 to 32 volts DC at a maximum load current of 70 amperes, and there were five TEC devices, each device drawing a peak current of 14 amperes at a voltage of 30 volts DC. The devices used were Model DL-290-24-00 sold by Supercool USA Inc. of San Rafael, Calif. A TEC heat pump control system for this type of application must therefore have the capability to deliver a large amount of power to multiple TEC devices. In addition, because such a system must be carried by a diver, the system weight and volume should be minimized in order to have minimum impact on diver mobility.
The three patents previously cited herein share a number of common characteristics, including the use of independent PWM energy conversion and polarity selection circuits, and the use of energy storage inductors to reduce the time-varying component of energy delivered to the TECs. In addition, the cited prior art teaches a single TEC device for each control system. While these characteristics may be advantageous for low-power applications as taught in the prior art, none of the cited prior art addresses the unique requirements of a man-portable system where multiple TEC devices operating in parallel are required in order to provide the required thermal energy transfer rates and where the power demand of each TEC device may exceed 200 watts. While it might be possible to realize a control system for this latter application in accordance with this prior art, the weight and volume of such a system would be untenable because of the number of components, especially with respect to the size and number of energy storage inductors required to realize a high-power multiple channel PWM energy conversion system.